This invention relates generally to the art of alloys and more particularly to an iron chromium silicon alloy having oxidation and corrosion resistance.
Chromium is the most commonly used addition to iron for improving iron's corrosion resistance. It is also well known that there is a critical amount of chromium (.gtoreq.9%) that is needed to make the alloy resistant to rusting. The most preferred amount of chromium is 18% that is the basis of the most abundantly used 300 Series of stainless if steels. The phase diagram in FIG. 1 shows several interesting aspects, a including: (a) iron and chromium are completely soluble in each other Hz over the entire range; and (b) over the major compositional range, the iron and chromium alloys form the body-centered-cubic (BCC) .alpha.-phase. However, there are a few exceptions to the simple behavior, including: (i) in the chromium level of 13%, the .alpha. phase at room temperature undergoes a crystal structure change from the BCC cc to face-centered-cubic (FCC) .gamma. in the temperature range of 831 to 1394.degree. C. The FCC .gamma. range can be stabilized to room temperature by increasing the chromium level to 18% and the nickel level to 8%. By converting the alloys to FCC over the entire temperature range, this provides at least three major benefits: (a) the FCC structure has higher strength up to high temperature of 1000.degree. C.; (b) such crystal structure is easy to fabricate at room and high temperatures; and (c) the FCC structure is free from the brittle to ductile transition (.ltoreq.room temperature to &gt;room temperature, respectively), which is a major issue with the BCC structure.
From the phase diagram in FIG. 1, it is also clear that the BCC .alpha.-phase is stable over the entire chromium content from .gtoreq.13 to 100% at temperatures exceeding 821.degree. C. However, at lower temperatures (function of chromium content), the BCC .alpha.-phase decomposes into a more complex .sigma.-phase. Thus, the iron-chromium alloys with chromium exceeding 8% show an .alpha.-+.sigma.-phase at room temperature. The presence of .alpha.-phase alone makes the alloy show: (a) lower strength that drops off rapidly at temperatures of .gtoreq.600.degree. C., and (b) the structure undergoes the brittle-to-ductile transition. The presence of .sigma.-phase makes the alloy more brittle.
Similar to chromium, silicon addition to iron also improves its oxidation resistance. The phase diagram of iron and silicon in FIG. 2 shows that at least up to 7% Si, the effect of silicon addition to iron is similar to that of the chromium addition to iron. The similarity is in stabilizing the FCC .gamma.-phase from 912 to 1394.degree. C. for a silicon content of 1.9%. Furthermore, the silicon addition up to approximately 7% stabilizes the BCC .alpha.-phase.
A combined look at the iron-chromium and iron-silicon phase diagrams indicates that the small additions of silicon (0 to 5%) to the iron-chromium system will only slightly alter its general crystal structural behavior.
From the above background, it is apparent that an Fe--Cr--Si alloy with good ii oxidation and corrosion resistance is desirable.